docs(user): document the per-viewer display time zone

[pat-s]

Document the time zone every log timestamp is rendered in, which comes from the viewer's browser and can be overridden in Profile → Time Zone.

Details

Rewritten to follow ricochet-rs/ricochet#1238, which shipped the feature. The earlier version of this PR documented a top-level timezone config key from ricochet-rs/ricochet#1233, which was closed unmerged. No such key exists on main, so the admin configuration page and its LinkCard are gone.

New page

src/content/docs/dev/user/time-zone.mdx, listed in the sidebar under User right after Quickstart, covers:

• Setting the zone from Profile → Time Zone, either by picking from the searchable list or pressing Detect.
• Precedence: the explicit choice, kept in a timezone cookie for a year, then the zone the browser reports, then UTC.
• That only an explicit choice is persisted, so a merely detected zone re-detects on a later visit from elsewhere, and that the choice lives in the browser rather than the account.
• What converts: the per-line clock in streamed runtime, task run, and deployment logs.
• What stays UTC and is labeled as such: run and deployment listings, execution environment validation times, the database, log files on disk, ricochet's own server logs, and audit records.
• That cron schedules are still evaluated in UTC, consistent with the existing note in Scheduling Tasks.
• That the zone list is read from the server's IANA database, and that an Alpine-based image needs tzdata for it to be available.

Verification

bun run build:astro completes, 270 pages build, and Starlight's link validation passes, including the new /dev/user/time-zone/ route and its outbound link to the scheduling page.
prek run --all-files is clean and scripts/semantic-breaks.mjs reports no changes needed.

[ricochet-bot]

Surge PR preview deployment succeeded. View it at https://ricochet-rs-docs-pr-312.surge.sh

[pat-s]

Merging so it's up-to-date with v0.17.0.